DESCRIPTION (from the application): The objective of this proposal is to better understand the mechanisms which govern human mesenchymal stem cells (hMSCs) in the bone marrow as progenitors in the pathway leading to osteoblasts. In particular, this proposal focuses on the individual and combined influence of two regulatory molecules, interleukin-1 (IL-1) and glucocorticoids (GCs), on regulating hMSC developmental activity and phenotypic expression. Both of these molecules are important regulators of bone homeostasis, however, the mechanisms of how these molecules effect bone development are very complex. Human bone marrow stroma contains a subpopulation of fibroblastic cells, hMSCs, that have the developmental potential to differentiate into multiple skeletal and connective cell types including osteoblasts, chondrocytes, adipocytes and stromal fibroblasts. The methods to isolate, culture-expand and induce hMSCs to differentiate along multiple phenotypic pathways have been documented. Extensive characterization of extracellular matrix, cell surface proteins and soluble factors synthesized by hMSCs have been reported. Data from these studies show that hMSCs, in addition to being progenitor cells of multiple phenotypes, are also regulatory cells of bone resorption and hematopoiesis. How the various developmental and metabolic activity of hMSCs are regulated in the bone marrow microenvironments is unclear. We have previously reported on the osteoinductive effects of the synthetic GC dexamethasone on hMSCs, and the stimulatory effects that IL-1 has on bone resorptive cytokine synthesis by these cells. Our recent data indicate that IL-1 enhances dexamethasone-induction of HMSC osteogenic differentiation. To better understand the role of hMSCs in the bone marrow as targets of these two regulatory agents, experimentation is planned to determine the mechanism(s) through which IL-1 enhances hMSC osteogenic differentiation in the presence of dexamethasone by investigating the roles of prostaglandins, integrins and interleukin-6-family cytokines in this induction. The data collected from this research plan will enhance our understanding of the local and systemic regulators of hMSCs, and strengthen our foundation of data for developing innovative approaches to clinical interventions, aimed at bringing imbalances in bone remodeling back into balance.